When utilizing a watchdog circuit for monitoring a pulse sequence of a microcomputer, the assumption is made that the microcomputer is "hung up" on a program step during the program sequence and the watchdog circuit emits a reset command to the microcomputer after a specific time has elapsed. This presupposes that the watchdog circuit always operates without malfunction. A watchdog circuit is disclosed in U.S. patent application Ser. No. 378,183, filed Jun. 26, 1989, now abandoned. The watchdog circuit of this application is disclosed in international patent publication WO 88/05570 and facilitates testing of actuating elements which are essentially computer controlled as well as the output of the microcomputer. However, a monitoring of the watchdog circuit itself does not take place.
A reliable monitoring of the watchdog circuit in combination with the microcomputer is especially required for systems which are relevant as to reliability and safety. Examples of such systems are ignition circuits or safety equipment in motor vehicles such as antilock systems or passive restraining systems such as an airbag or safety belts which must become operable at the instant of impact. However, since not only these systems but also the watchdog circuit itself can be subject to malfunction, it is required that their operational reliability be checked at specific time intervals and an alarm announcement be emitted in the case of a malfunction. A defect in the microcomputer or in the actuating element could no longer be detected when there is a breakdown of the monitoring arrangement, that is, of the watchdog circuit so that a malfunction can occur which is especially critical in a safety-relevant system.